In previous work with SiFAs, there was just one value associated with each parameter and each agent had a parameter as its target. Selectors or advisors provided the values for the parameters. Critics, praisers and evaluators worked with these values but the critiques, praises, or evaluations they produced were not stored separately. This is not very desirable if we wish to separate out all information.
The first extension to the SiFA model is to represent all value, estimate, criticism, praise, and evaluation entities separately, as first class objects. Being a first class object means that critiques, praises, and evaluations have the same status as the value of a parameter. They are all directly accessible and they can all be the target of an agent. This is done by having value, estimate, criticism, praise, and evaluation entities organized in a structure called the parameter block which is shown in figure 4.1.
Figure 4.1: The parameter block
The root of the parameter block is just the name of a parameter. The first level of reference has two entities. These are the value and the estimate of the parameter. There can be only one value and one estimate at any time.
The second level of reference has evaluations, criticisms, and praises of the value and the estimate. These entities are said to reference the value or the estimate of the value. There can be multiple criticisms, praises, and evaluations of the same value or estimate.
The third level of reference has evaluations, criticisms, and praises of second level evaluations, criticisms, and praises. These entities refer to the first level entities which refer to the first level entities. These chains of reference uniquely determine what each entity refers to. So at the third level, there exist entities such as the evaluation of the criticism of the value of the parameter. The third level entities typically contain meta-level information about the design, i.e., the evaluation of a criticism is not directly about the object being designed, but has an extra level of reference. Once again, there can be multiple evaluations, criticisms, and praises of the same second level entity.
It is also possible to imagine this structure growing into fourth, even fifth level references. Although what would be the contents of such entities might not be immediately obvious, the model allows such information to be represented if it is meaningful in any design problem and the experts of the domain wish to describe it.
These entities are stored on the central blackboard in the same way as the values of the design parameters that make up the current state of the design. They are accessible to all agents if they wish to use them.